An effusion study of gallium sesquiselenide, Ga2Se3, was conducted by simultaneous Knudsen effusion and torsion (Volmer) effusion in the range 1105-1289 K for the purpose of testing a theory of equilibrium condensed-phase transitions(1) in the presence of the vapor and of improving, on that basis, understanding of this important substance at high temperatures. Below 1225 +/- 10 K a phase nominally Ga2Se . Se-2(s) effused congruently, principally as Ga2Se(g) and Se-2(g), and above that temperature a phase nominally (Ga2Se)(1+x).(Se-2)(1-x)(1), x = 0.10, effused congruently as the same species; at 1225 +/- 10 K the vapor pressure was 14 +/- 1 Pa. Detailed observations of chemical events during the transition were made. A quadruple paint involving GaSe(s) was found at 1155 +/-5 K and 5 +/- 1 Pa. On plots of log P vs 1/T data fell in an envelope bounded above by curved three-phase lines in projection and below by straight two-phase lines, in conformity with theory. The Delta H degrees(298 K) of congruent vaporization of 1 mol of Ga2Se3(s) to give Ga2Se(g) and Se-2(g) was 649 +/- 3 kJ.